I investigated the variation in lifetime reproductive success (LRS) between males and females in the monogamous rodent Peromyscus californicus at the Hastings Natural History Reservation in central coastal California. Genealogical relationships of newly emerged juveniles were determined using a combination of fluorescent pigment transfer and DNA fingerprinting for 123 juveniles from June 1987 to April 1990. 2. Neither litter size nor interbirth interval changed in relation to parity (litter sequence), suggesting that reproductive success did not change with age. Interbirth intervals were significantly longer for individuals that re-mated compared to continuously mated individuals. 3. Lifetime reproductive success data were collected for 16 females and 20 males. As expected for a monogamous species, the mean and variance in lifetime reproductive success were statistically similar between males and females. 4. Maximum weight (g) and timing of first reproduction were significantly correlated with LRS in both males and females. None of the environmental features examined was significantly correlated with LRS in males or females. 5. Across all individuals, significant selection gradients were observed on timing of first reproduction in males, and maximum weight in females. Males that began breeding early produced more offspring than those breeding later due to increases in both survival and the number of litters produced. Heavy females produced more offspring than light females primarily due to their greater longevity (survival). 6. Across individuals with only one mate during their lifetime, significant phenotypic selection was observed only on timing of first reproduction in both males and females. 7. The mating system (monogamy) and reproductive patterns of P. californicus are distinct from most temperate Peromyscus. Monogamy in P. californicus does not appear to be a facultative response to the dispersion of females, but is due to increased reproductive success in males and females that invest in their pair bond rather than securing additional matings.